When producing hot dip galvannealed steel plate using a production facility of hot dip galvannealed steel plate, first, the steel plate is dipped in a plating bath filled with 440 to 480° C. molten zinc in a plating bath tank, then gas wiping nozzles spray the two surfaces of the steel plate with gas so as to adjust the plating deposition on the surfaces of the steel plate. Next, after adjusting the deposition, the steel plate is cooled to 400 to 460° C. or so, then heated again in an alloying furnace to 480 to 650° C. to make the iron in the steel plate and the deposited zinc react to thereby obtain an iron-zinc alloy plated steel plate. In general, the alloy layer of hot dip galvannealed steel plate is mainly comprised of the inferior sliding performance ζ-phase, superior sliding performance δ1-phase, and inferior adhesion Γ-phase. It is best to obtain an alloy layer mainly comprised of the superior sliding performance and adhesion δ1-phase.
The alloy phase formed by the alloying reaction differs depending on the temperature of the steel plate. It is known that the superior sliding performance and adhesion δ1-phase of steel plate is obtained near 490 to 650° C. In the conventional process of production of hot dip galvannealed steel plate, steel plate was heated in the alloying furnace (that is, the heating zone) of the alloying facility to 490 to 650° C., but the heating rate was slow, so the steel plate ended up being held for a long time at 470 to 490° C. (generally called the “ζ-phase forming temperature”) in the heating process. For this reason, a process of forming a large amount of ζ-phase at the steel plate surface, then transforming the ζ-phase to the δ1-phase was employed. In this case, the alloy crystals at the steel plate surface are mainly ζ-phase-derived needle crystals. At the surfaces of these large needle crystals, there are transformed small columnar crystals δ1. This steel plate surface is superior in sliding performance compared with a mainly ζ-phase surface, but is inferior in sliding performance compared with a mainly δ1 columnar crystal surface directly formed in the 490 to 650° C. temperature region, so is not desirable.
Further, in the process of ending the alloying reaction of the steel plate in the middle of the alloying facility or in the soaking zone at its exit, conventionally the steel plate had been air cooled, but the cooling rate is slow, so if the alloy layer surface is cooled after transforming to the δ1-phase, the bottom of the alloy layer transforms to the Γ-phase and the adhesion between the alloy layer and steel plate ends up deteriorating. Conversely, if the steel plate is cooled early so that the bottom of the alloy layer does not transform much to the Γ-phase, nonalloying defects of the surface occur and an optimum mainly δ1-phase alloy layer cannot be obtained.
To solve the above-mentioned problem, as technology for suppressing the formation of the ζ-phase at the alloy layer surface and the formation of the Γ-phase at the interface of the alloy layer and steel plate, the method of using an induction heating furnace etc. as the alloying furnace (that is, heating zone) of the alloying facility to raise the heating rate, the method of raising the cooling rate after soaking, the method of suitably controlling the plating deposition, the method of suitably controlling the Al concentration in the plating bath and in the plating layer, etc. have been researched.
For example, Japanese Patent No. 3,400,289 discloses, as an example of the optimum conditions to be applied to a conventional known alloying facility provided with a fixed type soaking zone and a fixed type cooling zone, the conditions of heating the steel plate by a 30° C./sec or higher heating rate, holding it at 470 to 510° C., and cooling it by a cooling rate of 30° C./sec or more until 420° C. or less. Further, Japanese Patent No. 2,848,074 discloses technology of an alloying facility able to switch between a movable type soaking zone and a movable type cooling zone and change a heat pattern. Furthermore, Japanese Patent Publication (A) No. 5-156419 discloses technology of an alloying facility provided with a furnace designed to switch between soaking and cooling. Further, Japanese Patent Publication (A) No. 63-121644 discloses technology of an alloying facility provided with a furnace designed to perform soaking by a heating gas and cooling by a cooling gas in the same region. Furthermore, Japanese Patent Publication (A) No. 2-122058 discloses technology of an alloying facility provided with a soaking region having feed ports of heating gas at the entry side of the steel plate and performing cooling as well in this soaking region. Specifically, this soaking region is divided into a plurality of zones, exhaust ducts for exhausting the atmosphere in a zone is set at the boundary of the zones, a cooling device is set in each zone, and soaking and cooling are selectively performed in each zone.